This invention belongs to the technical field of color CRT""s (color cathode ray tubes) (also referred to as color picture tubes) used in television receivers, displays, and the like. More specifically, this invention relates to a mask frame, a steel sheet used in manufacturing the mask frame and a manufacturing method therefor. The mask frame is a member which supports under tension a tension-type shadow mask which is disposed within a color CRT (in this specification referred to as a color CRT mask frame).
A color CRT has in its interior three electron guns for red, blue, and green and a fluorescent screen (screen) impacted by electron beams discharged therefrom. The surface of the fluorescent screen has fluorescent dots formed thereon which generate the above-mentioned three colors and which are arranged in a regular sequence.
In the type of shadow mask type used in the majority of color CRT""s, a rectangular shadow mask having a large number of aligned beam passage holes is disposed just in front of the fluorescent screen. The shadow mask is a member for performing alignment of the electron beams and the fluorescent dots so that each electron beam irradiates the fluorescent dots of the corresponding color.
A conventional ordinary shadow mask is made of a cold rolled steel sheet having a thickness of 0.15-0.28 mm in which fine holes with a regular spacing are formed by etching. After the shadow mask is bent by press forming, its four sides are welded to a mask frame and secured. The curvature of the mask is necessary so that thermal expansion of the mask and vibrations transmitted from the outside are absorbed by the mask and positional deviation of the holes in the mask does not take place. Accordingly, this type of mask can not adequately cope with flattening of the mask surface.
A more recently developed type is a tension-type shadow mask. A typical tension-type shadow mask is made of a thin steel sheet having a thickness of 0.05-0.15 mm in which small holes for the passage of beams are formed. It is attached to a mask frame in a state in which a tensile force is applied to it in the vertical direction. Thermal expansion and vibration of the mask can be absorbed by the tension, and the mask can be made flat. A tension-type shadow mask in which bi-directional tension is applied in both the vertical direction and the horizontal direction is also possible.
A mask frame for supporting the above-described typical tension-type shadow mask is normally assembled by welding two long-side frame members extending in the horizontal direction which form upper and lower frame portions and two short-side frame members extending in the vertical direction which form left and right frame portions. The long-side frame members are made from a steel sheet shaped by press forming or roll forming of the steel sheet. The thickness of the steel sheet is in the range of 3-6 mm, and it is selected in accordance with the size of the CRT. Round or rectangular pipes or bars are normally used as the short-side frame members.
Before attaching the shadow mask to the mask frame, blackening treatment of the mask frame is carried out. The blackening treatment is treatment in which a black film made of Fe3O4 is formed on the surface of the steel by heat treatment. The black film increases the thermal emissivity of the surface of the material, it increases the absorption and irradiation of electron beams, and it also has the effect of preventing the generation of secondary electrons and the formation of rust. The heating conditions for this blackening treatment are normally 450-680xc2x0 C. for 10-30 minutes.
Attaching the above-described tension-type shadow mask to a mask frame is carried out by welding the upper and lower edges of the shadow mask to the upper frame portion and the lower frame portion of the frame while compressing from the outside towards the inside the upper frame portion and the lower frame portion of the mask frame formed from the long-side frame members, and if necessary simultaneously applying tension to the shadow mask in the vertical direction. Then, when the pressure applied to the upper frame portion and the lower frame portion of the frame is removed, due to the rebound force of the frame, the shadow mask is supported by the frame in a state in which it is pulled in the vertical direction. The left and right edges of the shadow mask are not secured to the left and right frame portions of the mask frame (made from the short-side frame members).
In a structure in which a shadow mask is attached to a mask frame (also referred to below as a shadow mask/frame structure) in this manner, the upper frame portion and the lower frame portion of the mask frame are in a state in which a bending stress is applied, and the shadow mask is in a state in which it receives a tensile force in the vertical direction. The left and right frame portions of the mask frame perform the function of supporting the upper and lower frame portions which are under a bending stress.
Finally, stress relief annealing is applied to the shadow mask/frame structure, and strains occurring at the time of mask installation are removed. The stress relief annealing is generally carried out by heating at a temperature of 400-680xc2x0 C. for 10-30 minutes.
The order of steps of the above-described process (referred to below as Process A) is as follows:
Forming mask frame members-- greater than assembly of mask frame-- greater than blackening treatment of mask frame-- greater than mounting of shadow mask on frame-- greater than stress relief annealing
In the above order of steps, the step of blackening treatment and the step of stress relief annealing can be reversed. In this case, in the step of stress relief annealing, strains in the mask frame due to forming and welding are removed. In the step of blackening treatment, the mask frame and the shadow mask together undergo blackening treatment, and the strains resulting from installation of the shadow mask are also removed during the blackening treatment. The order of steps in this process (referred to below as Process B) is as follows:
Forming-- greater than assembly-- greater than stress relief annealing-- greater than installation of shadow mask-- greater than blackening treatment
In either of these processes, in the step of blackening treatment, it is desired to form a black film having good adhesion. If the adhesion of the black film is poor, there are cases in which the black film peels off during use of a color CRT, pieces of the black film fall down inside the CRT, the beam passage holes in the shadow mask are plugged and the like, and as a result, the image receiving properties of the CRT are greatly damaged.
The heat treatment step which is carried out after installation of the shadow mask (stress relief annealing in the case of Process A and blackening treatment in the case of Process B) is heat treatment carried out under the special circumstances in which the upper and lower frame portions of the mask frame are subjected to a bending stress and the shadow mask receives tension. When the bending stresses in the upper and lower frame portions of the mask frame are greatly alleviated by this heat treatment, there is the possibility of deformation of the mask frame taking place. A tensile force in the vertical direction is applied to the shadow mask by the mask frame, so deformation of the upper and lower frame portions of the mask frame causes a reduction in the tensile force acting on the shadow mask and causes surface strains. As a result, wrinkles and non-uniformity of the pitch of the holes develop, and are cases in which a deterioration of properties occurs such as impurity of color. Accordingly, it is important to suppress deformation of the mask frame during heat treatment.
In order to decrease the deformation of the mask frame during the above-described heat treatment step, which is a cause of a decrease in tensile force and surface strains of a tension-type shadow mask, the long-side frame members which make up the upper and lower frame portions of the mask frame which supports the shadow mask under tension have been manufactured from 36 Ni steel or 42 Ni steel having a high level of high-temperature creep strength. These steels respectively include 36% or 42% of Ni, which is expensive, so the mask frame becomes expensive.
Recently, televisions are tending to become large in size, and mask frames are also becoming large. In order to achieve decreases in the weight of televisions, there is a demand to increase the strength and decrease the thickness of steel which is used as a material for hanging mask frames.
An object of the present invention is to provide a steel sheet for a color CRT mask frame which is relatively inexpensive, which has excellent high-temperature strength, and which has a small amount of creep at high temperatures. As a result, deformation of a frame during heat treatment such as stress relief annealing carried out after mounting of a tension-type shadow mask on a mask frame can be suppressed to a minimum value, and the occurrence of wrinkles in a shadow mask and the occurrence of irregularities in the pitch of holes due to heat treatment can be prevented.
Another object of this invention is to provide a steel sheet for a color CRT mask frame which has a high strength so that the mask frame can be reduced in weight and which has adequate formability and on which a black film having good adhesion can be formed by blackening treatment.
Another object of this invention is to provide a method of manufacturing the above-described steel sheet for a color CRT mask frame.
The present invention is based on the below-described knowledge found by the present inventors.
(1) There is generally a tendency for deformation of a mask frame during heat treatment carried out under loading of a bending stress to be suppressed by using steel with a high level of high-temperature yield strength. However, even for steel materials having the same level of high-temperature yield strength, there is a considerable difference in the amount of deformation.
(2) With respect to deformation of the above-described mask frame, the degree of resistance to deformation of the steel can be determined by measuring the amount of creep occurring during one hour under conditions of a stress of at least 196 N/mm2 and a temperature of at least 400xc2x0 C.
(3) A steel sheet to which V and N are added in addition to Mo, which has the effect of improving high-temperature strength, and in which the ferrite grain size and the ferrite volume ratio are controlled to be at most prescribed values has a low value of creep under the above-described conditions even without containing a large amount of expensive Ni.
According to one aspect, the present invention is a steel sheet for a color CRT mask frame having a steel composition consisting essentially of, in mass %,
C: 0.03-0.30%, Si: at most 0.30%, Mn: 0.05-1.5%,
P: at most 0.05%, 5: at most 0.02%, Mo: 0.02-0.50%,
V: 0.02-0.20%, Al: at most 0.10%, N: 0.0040-0.0200%, PS optionally one or two or more of Cu: at most 1.0%, Ni: at most 1.0%, Cr: at most 2.0%, W: at most 1.0%, B: at most 0.003%, Ti: at most 0.030%, and Nb: at most 0.030%, and a balance of iron and unavoidable impurities, with Alxe2x89xa6(7.0)N, and having a metal structure in which the ferrite grain size is at most 15 micrometers and the ferrite volume ratio is at most 90%.
From another aspect, the present invention is a rectangular mask frame for a color CRT formed by joining four frame members, wherein at least a portion of the frame members is formed of the above-described steel sheets.
This invention also relates to a color CRT equipped with this mask frame.
According to another aspect, the present invention is a method of manufacturing a steel sheet for a color CRT mask frame including the following steps:
a step of manufacturing a slab having a steel composition consisting essentially of, in mass %,
C: 0.03-0.30%, Si: at most 0.30%, Mn: 0.05-1.5%,
P: at most 0.05%, S: at most 0.02%, Mo: 0.02-0.50%,
V: 0.02-0.20%, Al: at most 0.10%, N: 0.0040-0.0200%,
optionally one or two or more of Cu: at most 1.0%, Ni: at most 1.0%, Cr: at most 2.0%, W: at most 1.0%, B: at most 0.003%, Ti: at most 0.030%, and Nb: at most 0.030%,
and a balance of iron and unavoidable impurities, with Al less than (7.0)N, and
a step of hot rolling the slab under conditions of a finishing temperature of 820-950xc2x0 C. and a coiling temperature of 400-700xc2x0 C. to form a hot rolled steel sheet.
The above-described method may further include a step of carrying out cold rolling with a reduction of 0.2-15% of the hot rolled steel sheet obtained in the hot rolling step. In this case, it may further include a step of carrying out softening annealing of the hot rolled steel sheet at an annealing temperature of 600-750xc2x0 C. with a soaking time at the annealing temperature of 1-25 hours prior to the cold rolling step.
The present invention also provides a color CRT mask frame manufactured by a method including the following steps:
a step of shaping the above-described steel sheet to form a color CRT mask frame member,
a step of joining four mask frame members, at least a portion of which are the above-described mask frame member, to form a color CRT mask frame, and
a step of performing blackening treatment of the color CRT mask frame at a temperature in the range of 450-680xc2x0 C. to form a black film or a step of performing stress relief annealing of the mask frame at a temperature in the range of 400-680xc2x0 C.
The present invention also provides a tension-type color CRT shadow mask/frame structure manufactured from the above-described color CRT mask frame by a method including the following steps:
a step of securing a shadow mask to the above-described color CRT mask frame so that tension is applied to the mask to form a shadow mask/frame structure, and
a step of performing stress relief annealing of the structure at a temperature in the range of 400-680xc2x0 C. or a step of performing blackening treatment of the structure at a temperature in the range of 450-680xc2x0 C. to form a black film.
A steel sheet according to the present invention has excellent mechanical strength, but the room temperature and high-temperature mechanical strength and creep properties are further improved by precipitation of dissolved metal elements by the first heat treatment which is undergone (blackening treatment in Process A). For this reason, a mask frame which is manufactured from the steel sheet does not readily undergo deformation during heat treatment (stress relief annealing in Process A) which is carried out under the application of a bending stress after mounting of the shadow mask. Therefore, a decrease in tension of the shadow mask caused by this deformation can be suppressed to a minimum, and the generation of wrinkles and irregularity in the pitch of holes in the shadow mask due to the heat treatment are prevented.
In Japanese Published Unexamined Patent Application Hei 8-67945 (1996), a steel sheet for an aperture frame which supports an aperture grille, which is a shadow mask formed from a large number of ribbons is disclosed. It is explained that steel maintains a high level of high-temperature strength after stress relief annealing. However, the high-temperature strength in that case is the yield strength, and this is achieved by the addition of Mo. There is no suggestion whatsoever of the combined addition of V and N, as in the present invention, of the effect of ferrite structure, and of high-temperature creep properties.
A steel sheet according to the present invention is excellent not only with respect to high-temperature yield strength, but also with respect to high-temperature creep strength, and it exhibits low creep at high temperatures. In the following explanation, the terminology xe2x80x9chigh-temperature strengthxe2x80x9d will be used to include both high-temperature yield and high-temperature creep strength.